The global non-wovens market for polypropylene (PP) spunbond nonwoven (SBNW) materials is extremely large, with over 1700 kT of total global volume, split between market segments such as hygiene, homefurnishings, medical, industrial, etc. One of the most prominent property improvements desired for both absorbent hygiene materials and medical nonwovens produced from PP, is softness or haptics, in addition to noise and drape improvements. Polypropylene is the polymer of choice in the spunbond process due to its high tensile and abrasion resistance properties, the ease of processing, and the historically low price and high availability of the polymer. However, the haptics of the PP fabric are not ideal in terms of perceived softness.
Currently, there are a number of potential solutions for delivering softness or cloth-like feel for spunbond nonwovens. These include using bicomponent spunbond processes, using a blend of propylene/ethylene plastomers with PP, spinning random copolymers (that is random copolymers of polypropylene with 2-4% by weight of units derived from ethylene), and/or the addition of slip additives which can change the coefficient of friction (COF) of the PP surface. Additionally, there are fabrication modifications that can be implemented in order to change the surface of the fabric—thus making it feel softer. While these methods have proven successful to an extent, they have added cost or inefficiencies to the process. Accordingly new polypropylene materials which are capable of being spun into fiber in the spunbond process and produce soft fabrics are still desired.
A particular class of impact copolymers, which are historically considered to not be spinnable, has been discovered allowing at least some of these desired properties to be met. Accordingly, in one aspect of the present invention, an in-reactor polypropylene impact copolymer is provided which can be spun into fiber using the conventional spunbond process, and which will result in polypropylene fiber and formed fabric having improved softness. In one embodiment the invention is a polypropylene impact copolymer composition comprising from 60 to 90 percent by weight of the impact copolymer composition of a matrix phase comprising a homopolymer polypropylene or random polypropylene copolymer comprising from 0.1 to 7 mol percent of units derived from ethylene or C4-C10 alpha olefins; and from 10-40 percent by weight of the impact copolymer composition of a dispersed, preferably partially miscible phase comprising a propylene/alpha-olefin copolymer with alpha-olefin content ranging from 6-40 mol percent wherein the dispersed phase has a comonomer content which is greater than the comonomer content in the matrix phase. The difference should be sufficient, so that at least two distinct phases are present, although partial miscibility is desired. Although the specific amount that the comonomer must be different in order to ensure distinct phases will differ depending on the molecular weight of the polymers, in general it is preferred that the comonomer content in the dispersed phase is at least 10 mol percent greater (absolute), more preferably at least 12 mol percent greater. The impact copolymer of this embodiment is further characterized by having the ratio of the dispersed phase intrinsic viscosity (IV) to the matrix phase IV (also referred to as a beta/alpha value) being 0.95 or less.
A second aspect of the present invention is a fiber made from the impact copolymer of the first aspect of the invention. Such fibers can be melt spun on traditional spinning equipment to deniers of from 0.2 to 10, alternatively 0.5 to 2.0 and will have a broad bonding window.
Another aspect of the present invention is a spunbond nonwoven fabric produced from fibers of the second aspect of the invention. The spunbond nonwoven fabrics of this embodiment of the invention are characterized by having a lower bonding temperature as determined by the temperature of the calender oil being at least 5° C., preferably at least 10° C. lower than possible with a comparable nonwoven fabric made with hPP fibers; improved softness as determined by handle-o-meter and improved sensory testing panel results compared to nonwovens made with hPP fibers with regards to attributes such as smoothness, cloth-likeness, stiffness, and noise.